<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .app {
            width: 100%;
            height: 100%;
            background-color: #f5f5f5;
            position: relative;
            height: 500px;
        }
        .con{
            position: relative;
            height: 500px;
        }
        .real {
            position: absolute;
            height: 500px;
            width: 100%;
            top: 50%;
            left: 50%;
            transform: translate(-50%, -50%);
        }

    </style>
</head>
<body>
    <div class="app">
      <div class="con">
          <h1>hello world</h1>
      </div>
      <div class="real">
          <table>
              <tr>
                  <td>1</td>
                  <td>2</td>
              </tr>
          </table>
      </div>
    </div>
    
</body>
</html>